1. Field of the Invention
The present invention generally relates to a variable focal length lens system and an image capturing apparatus including the variable focal length lens system and, in particular, to a variable focal length lens system having a zoom ratio greater than 10 and an image capturing apparatus, such as a video camera or a digital still camera, including the variable focal length lens system.
2. Description of the Related Art
To record a subject image in cameras, a method for recording the subject image by converting the intensity of light of the subject image to an electrical output using an image pickup device including photoelectric transducers (such as charge coupled devices (CCDS) or complementary metal-oxide semiconductors (CMOSs)) has been known.
In recent years, microfabrication technology has been extensively developed. Accordingly, the speed of central processing units (CPUs) has been increased and the density of recording mediums has been increased. Thus, a large volume of image data that was unable to be processed before can now be processed at a high speed. In addition, the density of light receiving elements has been increased while the size of the light receiving elements has been decreased. The increased density of the light receiving elements allows a camera to record an image of high spatial frequency. The decreased size of the light receiving elements allows the size of the body of the camera to be decreased.
However, the increased density and decreased size of the light receiving elements reduce the light receiving area of each of the photoelectric transducers, and therefore, the intensity of the electrical output is decreased. Thus, the effect of noise on the electrical output increases. To reduce the effect of noise, an amount of light received by the light receiving element is increased by increasing the aperture ratio of an optical system or by mounting a microlens element immediately before each of the light receiving elements, forming what is known as a “microlens array”. Although the microlens array leads a light ray between the neighboring light receiving elements to the light receiving elements, the microlens array imposes a constraint on the position of the exit pupil of a lens system. When the position of the exit pupil of a lens system is close to the light receiving element, that is, when the angle between the light ray and the optical axis of a chief light ray that reaches the light receiving element is large, the angle between an off-axis ray propagating towards the periphery of a screen and the optical axis is made to be large. As a result, the off-axis ray does not reach the light receiving element, and therefore, an insufficient amount of light is received.
In recent years, as digital cameras have become more widely used, digital cameras have been required to meet a wide variety of user needs.
A zoom lens having a variable magnification ratio greater than 10 allows a camera to capture a significantly magnified image of a subject. In particular, since users of a fixed-lens camera cannot interchange lenses, some users require a lens system of a high variable magnification ratio even though the size of the body of the camera is increased.
For example, Japanese Unexamined Patent Application Publication No. 2005-215385, Japanese Unexamined Patent Application Publication No. 2003-295059, and Japanese Unexamined Patent Application Publication No. 2005-128186 describe zoom lenses having a variable magnification ratio more than 10.
The zoom lenses described in Japanese Unexamined Patent Application Publication Nos. 2005-215385 and 2003-295059 include a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power in this order from the object side. When the lens zoom position changes from a wide-angle position to a telephoto position, the first lens group is moved towards the object side, the second lens group is moved towards the image plane side, the third lens group is temporarily moved towards the object side and, subsequently, is moved towards the object side, and the fourth lens group is temporarily moved towards the object side and, subsequently, is moved towards the image plane side. In addition, in the zoom lens described in Japanese Unexamined Patent Application Publication No. 2005-215385, an aperture stop located between the second lens group and the third lens group can be moved independently from the other lens groups.
The zoom lens described in Japanese Unexamined Patent Application Publication No. 2005-128186 includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power in this order from the object side. When the lens position changes from a wide-angle position to a telephoto position, the first and third lens groups are stationary in the optical axis direction, the second lens group is moved towards the image-plane side, and the fourth lens group is moved so as to compensate for the change in the position of the image plane due to the movement of the second lens group.